Liquid-phase oxidation of an aromatic alkyl to an aromatic carboxylic acid is a highly exothermic chemical reaction. Typically, volatilizable aqueous solvents are employed to contain the reaction mixture and to dissipate heat of reaction.
Liquid-phase oxidation of aromatic alkyls to aromatic carboxylic acid conventionally takes place in a vented, well-mixed oxidation reactor equipped with an overhead condenser system. Such systems are shown in U.S. Pat. Nos. 3,170,768 and 3,092,658, both to Baldwin. A substantial portion of the reaction-generated heat is removed by evaporating a portion of the reaction mixture from the reactor, partially condensing it, and returning at least a portion of the condensate to the reactor.
In particular, an evaporated portion of the reaction mixture is withdrawn from the reactor head space. These vapors are then passed into an overhead condenser system that condenses a portion of these vapors and returns a resultant condensate stream to the reactor as reflux.
The non-condensed vapors that are discharged from the condenser system are then conventionally introduced into a shell-and-tube condenser of the knockback variety. In addition to condensing at least a portion of these condenser-system discharge vapors, the knockback condenser serves as a liquid-liquid separator to separate such condensed vapors into respective water-rich and solvent-rich phases. The solvent-rich phase is returned to the reactor as reflux. This solvent-rich reflux together with the earlier-mentioned condensate stream that is refluxed to the reactor from the overhead condenser system define a reflux loop.
Use of knockback condensers is undesirable for a variety of reasons. First, the liquid-liquid separator portion of the knockback condenser is provided with an internal baffle which separates a solvent-rich stream from a water-rich stream. Unscheduled process upsets or disturbances, which typically are costly, can arise when either one of the solvent-rich and water-rich streams carries over the baffle and combines with the other stream. Second, a knockback-type condenser can only accommodate a relatively limited gas velocity. That is, a gas velocity that is greater that a predetermined value typically gives rise to liquid entrainment into the knockback condenser overhead stream. Such entrainment has historically given rise to numerous process operating problems. Third, the thermal efficiency of a knockback condenser is not particularly desirable. For example, because of the relatively low gas throughput rate, heat transfer coefficients of a knockback type condenser are typically relatively low. Accordingly, there exists a need for other process system designs providing significantly higher heat transfer coefficients, which systems are able to provide relatively more efficient process control.
The present invention provides a method for continuously producing an aromatic carboxylic acid product that does not suffer from the foregoing problems. In the method of the present invention, the above-mentioned liquid-liquid separation step is not employed. Instead, the conventional knockback type of condenser is eliminated, a condenser having a significantly higher heat transfer coefficient can be utilized, and efficient control over the amount of water present in the reactor can be maintained. In particular, the present method enables utilizing a relatively more efficient condenser, such as a downflow-type condenser, in place of the knockback condenser. Because a condenser that does not function as a liquid-liquid separator can be used, upsets caused by liquid carryover are eliminated. The more efficient condenser, moreover, can be subjected to a relatively higher pressure drop than was allowable for the knockback type of condenser. This, in turn, permits relatively higher gas velocities through the condenser than had been possible employing the conventional knockback condenser.
In practicing the method of the present invention, the condenser produces a condensate that is relatively rich in water. A portion of the thus-produced water-rich condensate can be returned to the reactor feed, upstream of the oxidation reactor, to control water concentration within the oxidation reactor.